Two things bother me about the people laughing at ISIS involvement. 1)ISIS usually does not claim something unless they have some type of involvement even if it is just a little. 2)There is talk that the videos were live streamed to another party(if true that would say larger group).
They have claimed things that weren't true in the past.
If there was a connection between the shooter and ISIS and it turned out that he was a bad Muslim or said things he shouldn't have it could embarrass them.
Sometimes it's easier to just make things up.